One technique for detecting radiation is to use scintillation material to convert incident radiation into light. This light then can be measured by a light detector such as a photomultiplier tube. In applications where the location of the emitted radiation is important, as opposed to only determining the amount of radiation, a position-sensitive photomultiplier tube can be employed.
A position-sensitive photomultiplier tube converts light into electrons at a photocathode which is positioned across one end of the tube and multiplies the number of electrons through a series of parallel dynode meshes. The multiplied electrons impinge on one or more anode wires at the other end of the position-sensitive photomultiplier tube. Each anode wire is connect to one or more adjoining wires by resistors which form a resistor chain. When the electrons reach the anode wires the current at each end of the resistor chain changes. This current change is measured to pinpoint which anode wires are receiving the electrons. Accordingly, the corresponding position of the radiation which caused the light and subsequent electrons can be determined and displayed.
One application in which the field of view must be as large as possible is imaging radioactively labelled tissue for locating cancerous tumors in the body. This type of imaging has been performed with the imaging device scanning from outside the body. Since these newer imaging devices need to be placed in confined places, the size of the available field of view is particularly important.
One difficulty with the use of scintillators and position-sensitive photomultiplier tubes, however, is the edge effect. The edge effect is the loss of accurate position information for radiation received near the edges of the device. One cause of the edge effect is "edge packing". Edge packing occurs when the light emitted near the edge of the scintillation material is reflected off the edge of the scintillator to a position which does not correspond to where in the scintillation material the radiation was converted into light. Accordingly, this reflected light causes electrons to be generated and detected in the position-sensitive photomultiplier tube at positions different from where the radiation caused the light to be generated. For example, when part of the conical photon shower hits an insensitive area, the center of the mass of the registered signal is moved to the center of the position-sensitive photomultiplier tube.
One proposed solution to the edge packing problem is discussed in U.S. Pat. No. 4,990,785 to Logan. Logan discusses the placement of additional photomultipliers at the edge of the scintillation material so that the light will not be reflected, but instead will be measured separately. However, providing additional photomultiplier tubes increases the costs and size of the device. Size should be as small as possible for intraoperative use. Another proposed solution is to prevent the incident radiation from impinging on the edges of the scintillators by using collimators lacking holes near the edges, see for example, U.S. Pat. No. 3,668,395 to Walker, or by using collimators with long septa, see for example U.S. Pat. No. 4,180,737 to Kingsley. However, both these proposed solutions reduce the field of view by blocking some potentially non-reflecting radiation that would otherwise be received. Additionally, these solutions reduce the sensitivity of the camera by increasing the time required to form an image. Camera sensitivity is critically important for intraoperative applications since time is limited during surgery.
Another edge effect is due to current leakage within the position-sensitive photomultiplier tube. Specifically, some of the electrons leak out of the position-sensitive photomultiplier tube near the sides of the position-sensitive photomultiplier tube. Accordingly, since fewer electrons reach the anode wires which are closer to the edge of the position-sensitive photomultiplier tube than reach the other anode wires for the same amount of corresponding radiation, a distorted image which does not accurately reflect the received radiation near the edge of the device is displayed.
Although solutions to distorted images caused by other effects have been proposed, see for example U.S. Pat. No. 4,292,538 to Carlson, U.S. Pat. No. 4,398,092 to Carlson and U.S. Pat. No. 4,316,257 to Del Medico, the Carlson patents only discuss such correction for multiple detectors and Del Medico only discusses a software solution to correct for distortion.